Yes. You can buy PlusBus day tickets at self-service ticketing machines at stations managed by the following train companies:-
Greater Anglia, Great Western Railway, East Midlands Trains, South Western Railway, Southern, Southeastern and Virgin Trains.
Self-service ticket machines at main stations managed by Virgin Trains East Coast and First TransPennine Express also sell PlusBus.
To see how to serve yourself with a PlusBusday ticket, click here.